A multicellular organism is composed of individual cells that are organized structurally and functionally into tissues, organs, and systems to conduct various activities of the organism. For example, muscle cells have been recognized as functionally involved in movement while liver cells have been identified as removing toxic substances. There remain many instances, however, where it is difficult to identify the individual cells involved in a particular activity or to identify subsets of cells and their different roles during a complex activity. The inability to identify individual cells, as opposed to a region of cells, as involved in various brain functions is one example of the continuing difficulties in the field.
The central nervous system of mammals is composed of millions of neurons that are connected to each other in a wide variety of highly specific combinations. Individual neurons have distinct developmental programs, different patterns of connectivity with other neurons, and restricted anatomical locations. To complicate things further, neurons alter their functional properties in an experience-dependent way. The key determinants of a given neuron, such as its functional properties and it connections to other cells, thus change over time and in response to various stimuli.
In order to determine the neural basis for behavior and memory, scientists have employed a variety of techniques to allow the identification of brain areas, and thus particular neurons, that may be involved. For example, tissue lesion, either by stroke, surgery or toxin exposure, has been used to verify the involvement of a region of the central nervous system (CNS) in a particular behavioral task or response. This has been a fruitful method in some respects, as it has allowed identification of the hippocampus, for example, as a site of short-term memory storage, the hypothalamus as an area involved in appetite and thirst regulation, and the cerebellum as an area involved in motor planning and coordination. Lesion analysis has not provided a direct test of functional involvement, however, and usually involves the disruption of an enormous number of neurons without identification of which neurons are actually involved in a particular behavior. Many of the neurons in an identified CNS region may not be involved in the behavior being assayed.
In a similar attempt to uncover the neural basis of memory and behavior, studies have attempted to use assays of single neurons in vivo in an attempt to look for changes in neuronal firing properties after training or during a particular behavioral state. (Thomas M J, Beurrier C, Bonci A, Malenka R C. Long-term depression in the nucleus accumbens: a neural correlate of behavioral sensitization to cocaine. Nat Neurosci. 2001 December; 4(12):1217-23.) This technology is limited, however, in that it is difficult to distinguish the activated subset of neurons (signal) that are directly involved in a behavior from those that are not (noise). For this reason, sophisticated statistics and large data sets are required to detect any changes between trained and untrained animals, for example.
In another study, a genetic approach was used to visualize axons from a subset of olfactory sensory neurons as they project to the olfactory bulb (Mombaerts et al. Cell 87(4):675-686, 1996). Additional projections of neurons have been charted by the use of a variety of axon-tracing techniques (Callahan et al. Curr. Opin. Neurobiol. 8(5):582-586, 1998).
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